Piezoelectric thin film resonator and manufacturing method thereof

ABSTRACT

A piezoelectric thin film resonator which can reduce variations in resonant frequency and resonant resistance by uniformly planarizing a structural film, and a method of manufacturing the piezoelectric thin film resonator. The piezoelectric thin film resonator has a substrate having at least one flat major surface; a dielectric film having two support portions supported by the major surface of the substrate and a floating portion which is connected to the support portions and which is disposed over the major surface of the substrate with an airspace layer provided therebetween; and a vibration portion which is formed of a pair of electrodes and a piezoelectric thin film provided therebetween and which is provided on the floating portion of the dielectric film at a side opposite to the airspace layer. A surface of the dielectric film at a side opposite to the substrate is planarized by a plasma treatment using an inert gas or a gas containing an element forming a dielectric film.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International ApplicationNo. PCT/JP2005/022300, filed Dec. 5, 2005, which claims priority toJapanese Patent Application No. JP2004-373761, filed Dec. 24, 2004, theentire contents of each of these applications being incorporated hereinby reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a piezoelectric thin film resonator anda manufacturing method thereof.

BACKGROUND OF THE INVENTION

In a so-called air-bridge type piezoelectric thin film resonator, inorder to acoustically separate a vibration portion, which is formed of apair of excitation electrodes facing each other and a piezoelectric thinfilm provided therebetween, from a substrate, the structure is formed inwhich a thin film member (membrane) partly floats over the substratewith an airspace layer provided there between. Patent Document 1:Japanese Unexamined Patent Application Publication No. 61-218214

Heretofore, planarization has not been studied for an air-bridge typepiezoelectric thin film resonator.

The orientation property of a piezoelectric thin film influencesresonant properties. In order to improve the resonant properties, theorientation property of a piezoelectric thin film must be improved, andin order to improve the orientation property thereof, the flatness of astructural film, which is provided under an excitation electrode, mustbe improved.

As a method for planarizing the structural film provided under theexcitation electrode, for example, polishing by CMP (chemical mechanicalpolishing) may be mentioned.

In an air-bridge type piezoelectric thin film resonator, a structuralfilm is formed on a sacrifice layer which is used for forming anairspace layer. Hence, irregularities are formed on the structural filmwhich is to be polished. By polishing, convex portions (projectingsurfaces) can only be planarized, and in addition, since slurry istrapped in concave portions, peripheries of the convex portions arelikely to be polished as compared to central portions thereof.Accordingly, even when the structural film on the sacrifice layer ispolished, film-thickness distribution is generated, and as a result,superior resonant properties cannot be obtained. In order to avoid thisproblem, a method may be used in which after a burying material isburied in concave portions, planarization is performed by CMP, and theburying material is then removed; however, in this case, themanufacturing process becomes complicated, and hence the manufacturingcost is increased.

In addition, when a plurality of piezoelectric thin film resonators issimultaneously formed using a wafer, it is difficult to uniformly polishstructural films of individual resonators in the wafer on the order ofnanometers in thickness. The resonant frequency and the resonantproperties of a resonator largely depend on the thickness of thestructural film. When the thicknesses of the structural films varywithin a wafer, the yield may be decreased, or the number of steps, suchas frequency adjustment, may be increased, and as a result, themanufacturing cost is increased.

SUMMARY OF THE INVENTION

The present invention has been conceived in consideration of the abovecircumstances, and an object of the present invention is to provide apiezoelectric thin film resonator having superior resonant properties byuniformly planarizing a structural film and a manufacturing method ofthe above piezoelectric thin film resonator.

In order to achieve the above object, the present invention provides apiezoelectric thin film resonator having the following structure.

The piezoelectric thin film resonator includes a substrate having atleast one flat major surface; a dielectric film having two supportportions supported by the major surface of the substrate and a floatingportion which is connected to the support portions and which is disposedover the major surface of the substrate with an airspace layer providedtherebetween; and a vibration portion which is formed of a pair ofelectrodes and a piezoelectric thin film provided therebetween and whichis provided on the floating portion of the dielectric film at a sideopposite to the airspace layer. In this piezoelectric thin filmresonator, a surface of the dielectric film which is located at a sideopposite to the substrate is planarized by a plasma treatment using aninert gas or a gas containing an element forming the dielectric film.

According to the structure described above, since the dielectric film isplanarized by a plasma treatment, the orientation property of theexcitation electrode formed on the dielectric film is improved, andhence the orientation property of the piezoelectric thin film formed onthe excitation electrode is also improved. Since the orientationproperty of the piezoelectric thin film is improved, variations inresonant properties and resonant resistance can be reduced. In addition,since the orientation property of the excitation electrode is improved,a piezoelectric thin film resonator having superior resistance againstelectric power can be formed.

In addition, when a plurality of piezoelectric thin film resonators issimultaneously formed using a wafer, the thickness of the structuralfilm of each resonator can be controlled on the order of nanometers.Accordingly, since variation in resonant frequency in the wafer surfaceand the resonant properties can be controlled with high accuracy, theyield can be increased, and hence the manufacturing cost can be reduced.

Furthermore, since a plasma treatment is performed using an inert gas ora gas containing an element forming the dielectric film, the dielectricfilm can be planarized without forming a compound layer thereon which isdifferent from a dielectric material.

The dielectric film is preferably a film formed of a material selectedfrom the group consisting of Si₃N₄, SiO₂, and Al₂O₃.

According to the above structure, since the dielectric film is anamorphous film, planarization can be easily performed by a plasmatreatment.

In addition, the present invention provides the following method formanufacturing piezoelectric thin film resonators.

The method for manufacturing piezoelectric thin film resonators,includes: i) a first step of forming sacrifice layer patterns on anupper surface of a mother substrate; ii) a second step of forming adielectric film on the sacrifice layer patterns; iii) a third step ofprocessing a surface of the dielectric film by a plasma treatment; iv) afourth step of forming vibration portions on the dielectric film, thevibration portions each being composed of two excitation electrodes anda piezoelectric thin film provided therebetween; v) a fifth step ofetching the sacrifice layers; and vi) a sixth step of cutting the mothersubstrate to separate the piezoelectric thin film resonators.

Since the dielectric film is planarized in the above third step, theorientation property of the excitation electrode formed on thedielectric film is improved, and the orientation property of thepiezoelectric thin film formed on the excitation electrode is alsoimproved. Since the orientation property of the piezoelectric thin filmis improved, a piezoelectric thin film resonator having superiorresonant properties can be manufactured.

The third step described above preferably includes the substeps of: a)fitting the mother substrate provided with the dielectric film formedthereon to a substrate plate and then placing the mother substrate in asputtering chamber; b) supplying a gas in the sputtering chamber; and c)performing sputtering etching of the surface of the dielectric film,which is formed on the mother substrate fitted to the substrate plate,with plasma of the gas generated by supplying an RF voltage to thesubstrate plate while the substrate plate is being electrically floatedfrom the sputtering chamber.

In this case, in the third step, the dielectric film can be planarizedwithout forming a compound layer thereon which is different from adielectric material. In addition, a dielectric film patterning step canbe stably performed when etching holes are formed.

In particular, the following embodiments may be mentioned.

As one embodiment, the gas may be an inert gas selected from the groupconsisting of Ar and He.

As another embodiment, the gas may be a gas containing an elementforming the dielectric film.

In said another embodiment, the dielectric film is preferably formed ofSiO₂ or Al₂O₃, and oxygen is preferably used as the gas. In this case,since the insulating properties are excellent, the resonant propertiesare not degraded.

In said another embodiment, the dielectric film is preferably formed ofSi₃N₄, and nitrogen is preferably used as the gas. In this case, sincethe insulating properties are excellent, the resonant properties are notdegraded.

According to the piezoelectric thin film resonator and the manufacturingmethod thereof, by uniformly planarizing the structural film, aresonator having superior resonant properties can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a cross-sectional view of a piezoelectric thin filmresonator, and FIG. 1(b) is a plan view thereof (example).

FIG. 2 is a plan view of a piezoelectric thin film resonator, whichcorresponds to FIG. 1(b) (modified example).

REFERENCE NUMERALS

10, 10 a piezoelectric thin film resonator

11 substrate

13 airspace layer

14 lower electrode (excitation electrode)

16 piezoelectric thin film

17 sacrifice layer

17 x end portion

18 upper electrode (excitation electrode)

19 vibration portion

DESCRIPTION OF THE INVENTION

hereinafter, examples will be described as embodiments according to thepresent invention with reference to FIG. 1. FIG. 1(a) is across-sectional view taken along the line A-A in FIG. 1(b), and FIG.1(b) is a plan view.

As schematically shown in FIG. 1, a piezoelectric thin film resonator 10is formed of a substrate 11 and a thin film member (membrane) providedthereon, the thin film member including a dielectric film 12, a lowerelectrode 14, a piezoelectric thin film 16, and an upper electrode 18.An airspace layer 13 (see FIG. 1(a)) is formed between the substrate 11and the dielectric film 12. The dielectric film 12 includes supportportions supported by the substrate 11 and a floating portion floatingover the substrate 11. On the floating portion of the dielectric film 12at a position opposite to the airspace layer 13, a vibration portion 19floating over the substrate 11 is formed in a region in which theelectrodes 14 and 18 are overlapped with each other. The vibrationportion 19 is composed of parts of the lower electrode 14, thepiezoelectric thin film 16, and the upper electrode 18, which arepresent in the above region. The airspace layer 13 is formed by removinga sacrifice layer 17 (see FIG. 1(b)) which is formed between thesubstrate 11 and the dielectric film 12.

Next, a manufacturing method of the piezoelectric thin film resonator 10will be described.

<Sacrifice Layer>

First, the sacrifice layer 17 is formed on the substrate 11. As thesubstrate 11, a substrate which is inexpensive and which has superiormachinability is used. A Si or a glass substrate having a flat surfaceis more preferable. On this substrate 11, by methods such as asputtering method and a photo-etching method, the sacrifice layer 17 forforming an airspace layer is formed using, for example, zinc oxide whichis likely to be chemically dissolved.

As a material for the sacrifice layer 17, it is preferably used thematerial can withstand a high temperature generated when thepiezoelectric thin film 16 is formed, and which can be easily removed.For example, a metal such as Ge, Sb, Ti, Al, or Cu, a malate silicateglass (PSG), or a polymer may be used. As the polymer, for example,polytetrafluoroethylene or its derivative, poly(phenylene sulfide),poly(ether ether ketone), polyimide, poly(imide siloxane), vinyl ether,polyphenyl, parylene-n, parylene-f, or benzocyclobutene is preferable.

The sacrifice layer 17 should have a sufficient thickness so that thevibration portion 19 is not brought into contact with the substrate 11even when the membrane is warped. For easy formation, the thickness ofthe sacrifice layer 17 is preferably in the range of 50 nm to severalmicrometers. In addition, the minimum distance between the vibrationportion 19 and an end portion 17 x of the sacrifice layer 17 is set tobe not more than 50 times the thickness of the vibration portion 19.

<Formation of Dielectric Film>

Next, the dielectric film 12 is formed. The dielectric film 12 is formedso as to cover the entire surface of the substrate 11 by a method suchas sputtering, CVD, or electron beam deposition. This dielectric film 12has an effect of protecting the vibration portion 19, which includes theelectrodes 14 and 18 and the piezoelectric thin film 16, and may beformed using a nitride such as silicon nitride having superiorpassivation properties or an oxide such as silicon oxide.

In addition, for this dielectric film 12, when a material having atemperature coefficient of frequency (TCF) opposite to that of amaterial used for the piezoelectric thin film 16 is used, the change infrequency with respect to the change in temperature of a resonator or afilter is decreased, and hence the properties are improved. For example,when zinc oxide or aluminum nitride is used for the piezoelectric thinfilm, silicon oxide having a TCF opposite to that thereof may be used.

In addition, aluminum nitride which is an insulating material and whichhas high thermal conductivity may also be used.

This dielectric film 12 is planarized in a subsequent step. As amaterial suitable for the planarization, an insulating film may be used,and as a preferable material, for example, an amorphous material such assilicon nitride, silicon oxide, or aluminum oxide may be used.

<Planarization of Dielectric Film>

Next, the dielectric film 12 is planarized. That is, the dielectric film12 is planarized by dry etching.

As the dry etching, either ion etching or plasma etching may beperformed. In the case of ion etching, an inert gas such as Ar or He isdischarged by RF power, and sputtering etching is performed byself-bias, so that planarization may be performed. That is, by plasma(electrons and positive ions) generated by supplying an RF voltage to asubstrate plate (electrically floating from a sputtering chamber) in thesputtering chamber, the substrate plate is biased to a negativepotential with respect to a standard potential, so that sputteringetching is performed. In the case in which the dielectric material is anoxide such as silicon oxide, an oxygen gas may be used. Positive ionsare Ar⁺ ions when the gas is argon and are O⁺, O₂ ⁺ (two-atom ion) andthe like when the gas is oxygen. In addition, planarization may beperformed by reactive ion etching using a chemically active gas such asa halogen compound. Since a plasma treatment is performed using an inertgas or a gas containing an element forming a dielectric film, thedielectric film 12 can be planarized without forming a compound layerthereon which is different from a dielectric material.

A preferable surface roughness (Ra) of the dielectric film 12 is 2.0 nmor less. This surface roughness (Ra) is called an arithmetic averageroughness and is an average value obtained such that a standard length Lis extracted from a roughness measurement curve in a direction of itsaverage line, and the absolute values of deviations from the averageline of this extracted portion to a measurement curve are summed andaveraged.

As one example of plasma treatment conditions, when an SiO₂ film isprocessed at an RF power of 6 mW/mm² for a treatment time of 10 minutesusing an O₂ gas, an initial surface roughness Ra of 2 to 10 nm isimproved to a surface roughness Ra of 2 nm or less by the treatment.

SiO₂ is obtained as an amorphous material by common sputtering. Evenwhen the same treatment as described above is performed for a ZnO filmwhich is likely to be uniaxially oriented, the surface roughness is notso much improved.

<Formation of Lower Electrode>

Next, the lower electrode 14 is formed on the dielectric film 12processed by the planarization treatment. The lower electrode 14 isformed by film formation using sputtering, plating, CVD, electron beamdeposition or the like, followed by patterning using a photolithographictechnique. The lower electrode 14 is primarily formed from a metalmaterial, such as Mo, Pt, Al, Au, Cu, or Ti, to have a belt shapeextending from the sacrifice layer 17 to one side of the substrate 11(right side in the figure). Since the dielectric film 12, which is alayer provided below the lower electrode 14, is planarized, the lowerelectrode 14 can be formed to have a flat surface. A preferable surfaceroughness (Ra) of the lower electrode 14 is 2.0 nm or less.

<Formation of Piezoelectric Thin Film>

Next, the piezoelectric thin film 16 is formed on the lower electrode14. By film formation using sputtering or the like and by lift-off usingpatterning by a photolithographic technique, the piezoelectric thin film16 is formed using zinc oxide, aluminum nitride, or the like. Whenaluminum nitride is used for forming the piezoelectric thin film 16, bylift-off using zinc oxide, aluminum nitride is patterned. Since thedielectric film 12 of silicon oxide or the like is formed over theentire surface of the sacrifice layer 17 formed of zinc oxide, althoughzinc oxide used for lift-off is wet-etched when it is patterned or whenaluminum nitride is processed by lift-off, zinc oxide used for formingthe sacrifice layer 17 is not etched.

<Formation of Upper Electrode>

Next, the upper electrode 18 is formed. The upper electrode 18 is formedon the piezoelectric thin film 16 in a manner similar to that for thelower electrode 14. The upper electrode 18 is formed to have a beltshape extending from the piezoelectric thin film 16 to the other side ofthe substrate 11 (left side in the figure).

<Formation of Etching Hole>

Next, etching holes, which are penetrating portions for exposing thesacrifice layer 17, are formed. After a photoresist or the like ispatterned by photolithography, by reactive ion etching (RIE), wetetching, or the like, parts of the dielectric film 12 provided on thesacrifice layer 17, which are not covered with a photoresist pattern,are removed. A photoresist pattern shown in FIG. 1(b) has a rectangularshape covering the lower electrode 14, the piezoelectric thin film 16,and the upper electrode 18. The end portions 17 x of the sacrifice layer17 under the insulating film 12 are extended outside from the resistpattern. The photoresist pattern may have a cross-shape as shown in FIG.2. For example, when silicon oxide is used for the dielectric film 12,reactive ion etching is performed using a fluorinated gas such as CF₄.Alternatively, wet etching may be performed using a hydrofluoric acidsolution. After the etching, an etching mask such as a photoresist isremoved using an organic solvent such as acetone. Dry etching usingoxygen plasma may also be performed.

<Formation of Airspace Layer>

Next, the sacrifice layer 17 is etched, so that the airspace layer 13 isformed. After a photoresist or the like is patterned byphotolithography, by reactive ion etching, wet etching, or the like, thesacrifice layer 17 is removed. For example, when zinc oxide is used forforming the sacrifice layer 17, it is removed using an acidic solutioncontaining hydrochloric acid, phosphoric acid, or the like. After theetching, an etching mask such as a photoresist is removed using anorganic solvent such as acetone. When the sacrifice layer 17 is etchedusing a solution which does not etch the piezoelectric thin film 16, thedielectric film 12, and the electrodes 14 and 18, a process includingpatterning by photolithography and removal of an etching mask can beomitted. For example, when aluminum nitride is used for thepiezoelectric thin film 16, silicon oxide is used for the dielectricfilm 12, and Pt, Au, Ti, or the like is used for the electrodes 14 and18, zinc oxide forming the sacrifice layer 17 can be removed by a mixedaqueous solution composed, for example, of acetic acid and phosphoricacid without performing patterning. After the etching, replacement usinga volatile solution such as pure water or IPA is sufficiently performed,followed by drying, so that the airspace layer 13 is formed.

When mass production of the piezoelectric thin film resonator 10 isperformed, the piezoelectric thin film resonators 10 are simultaneouslyformed by the above manufacturing method using a wafer (mothersubstrate) as the substrate 11 and are then separated by dicing or thelike, so that individual piezoelectric thin film resonators 10 areobtained. Alternatively, after a packaging substrate having lands isprepared, and the upper and the lower electrodes of the mother substrateare bonded to the lands by bump-bonding before individual piezoelectricthin film resonators are separated by cutting, the peripheries of thepiezoelectric thin film resonators may be encapsulated for packaging.

The piezoelectric thin film resonator 10 thus described has thefollowing operations and advantages.

(1) Since the dielectric film 12 used as an underlayer provided belowthe lower electrode 14 is planarized, the lower electrode 14 can beformed to be flat and to have a superior orientation property, and thepiezoelectric thin film 16 having superior quality can be formedthereon; hence, the piezoelectric thin film resonator 10 having superiorproperties can be obtained. In addition, since the orientation propertyof the lower electrode 14 is improved, the piezoelectric thin filmresonator 10 can be formed to have superior resistance against electricpower.

(2) Since a dry process is used for planarization instead of CMP, thethickness of the dielectric film 12, which is used as an underlayerprovided below the lower electrode 14, can be uniformly controlled onthe order of nanometers in the substrate surface. Accordingly, since thevariation in resonant frequency in the wafer surface and the resonantproperties can be controlled with high accuracy, the yield can beincreased, and hence the manufacturing cost can be reduced.

That is, the resonant frequency and the resonant properties of aresonator considerably depend on the thickness of its constituent film.When the thickness of the constituent film varies in a wafer surface,the yield may be decreased, and/or the number of steps such as frequencyadjustment may be increased; hence, as a result, the manufacturing costis increased. When a plurality of resonators is simultaneously formedusing a wafer, the thickness of the constituent film of each resonatorcan be controlled on the order of nanometers by a plasma treatment, andhence the variation in resonant frequency in the wafer surface and theresonant properties can be controlled with high accuracy. As a result,the yield can be increased, and the manufacturing cost can be reduced.

(3) In the case in which zinc oxide is used both for the sacrifice layer17 for forming the airspace layer 13 and a lift-off mask used when thepiezoelectric thin film 16 made of aluminum nitride is formed, when thedielectric film 12 is formed over the entire surface of the sacrificelayer 17 when aluminum nitride is patterned, the shape of the sacrificelayer 17 is not damaged when the aluminum nitride is processed bylift-off.

(4) When structural films other than the sacrifice layer 17 haveresistance against an etching solution for the sacrifice layer 17, apatterning step for sacrifice-layer etching can be omitted, and as aresult, because of process stabilization and decrease in number ofsteps, the cost can be reduced.

(5) After the sacrifice layer 17 is wet-etched, the etchant issufficiently replaced with a volatile solution such as pure water orIPA. When the replacement is performed using a volatile solution, timerequired for a drying step following the removal of the sacrifice layercan be reduced, and hence the cost can be reduced.

The present invention is not limited to the above examples, and variousmodifications may be performed without departing from the scope of thepresent invention. The present invention may also be applied to a laddertype and a lattice type piezoelectric filter using a plurality ofpiezoelectric thin films.

1. A piezoelectric thin film resonator comprising: a substrate having atleast one flat major surface; a dielectric film having two supportportions supported by the major surface of the substrate and a floatingportion connected to the support portions and disposed over the majorsurface of the substrate with an airspace layer provided therebetween;and a vibration portion comprising a pair of electrodes and apiezoelectric thin film provided therebetween, the vibration portionbeing provided on the floating portion of the dielectric film at aposition opposite to the airspace layer, wherein a surface of thedielectric film opposite to the substrate is plasma treatment planarizedusing an inert gas or a gas containing an element forming the dielectricfilm.
 2. The piezoelectric thin film resonator according to claim 1,wherein the dielectric film comprises a material selected from the groupconsisting of Si₃N₄, SiO₂, and Al₂O₃.
 3. A method for manufacturingpiezoelectric thin film resonators, the method comprising: formingsacrifice layer patterns on an upper surface of a mother substrate;forming a dielectric film on the sacrifice layer patterns; processing asurface of the dielectric film by a plasma treatment; forming vibrationportions on the dielectric film, the vibration portions each beingcomposed of two excitation electrodes and a piezoelectric thin filmprovided therebetween; etching the sacrifice layers; and cutting themother substrate into separate piezoelectric thin film resonators. 4.The method for manufacturing a piezoelectric thin film, according toclaim 3, wherein the step of processing the surface of the dielectricfilm comprises: fitting the mother substrate provided with thedielectric film formed thereon to a substrate plate; placing the mothersubstrate in a sputtering chamber; supplying a gas in the sputteringchamber; and performing sputtering etching of the surface of thedielectric film with plasma of the gas generated by supplying an RFvoltage to the substrate plate while the substrate plate is electricallyfloated from the sputtering chamber.
 5. The method for manufacturing apiezoelectric thin film, according to claim 4, wherein the gas is aninert gas selected from the group consisting of Ar and He.
 6. The methodfor manufacturing a piezoelectric thin film, according to claim 4,wherein the gas is a gas comprising an element forming the dielectricfilm.
 7. The method for manufacturing a piezoelectric thin film,according to claim 6, wherein the dielectric film is formed of SiO₂ orAl₂O₃, and oxygen is used as the gas.
 8. The method for manufacturing apiezoelectric thin film, according to claim 6, wherein the dielectricfilm is formed of Si₃N₄, and nitrogen is used as the gas.